Assume a GOP Senate in '21-'23. If you're Justice Breyer, I would think a sensible strategy would be to announce your retirement soon-ish, but make it "effective upon the nomination and confirmation of my successor," as Justice O'Connor did.
https://www.npr.org/2020/12/29/950654338/religion-abortion-guns-and-race-just-the-start-of-a-new-supreme-court-menu

That way, if Biden can get a replacement confirmed this Senate, the new Justice will come in and Breyer will step down; if Biden can't get a replacement confirmed, however, Breyer will stay on. Assuming Breyer is ready to retire, that would seem to make some sense, I think.

I should add that I suspect Biden would nominate Leondra Kruger, who is very widely admired and has avoided having any hard-edged ideological positions. I would think she would be readily confirmed. But for Breyer, this seems a sensible strategy, I would think.

So to make it interesting, here's a prediction: Breyer will announce his retirement, to be effective when a replacement is confirmed, sometime this spring or (latest) at the end of this Term.